Micro-/macro-scale Water And Gas Transport Properties And Practical Performance Of Capillary Barrier Covers

Substantial theoretical and experimental studies have demonstrated that capillary barrier covers(CBCs)perform well at limiting percolation in arid,semi-arid and some semi-humid regions in North America.Loess is widely distributed in Northwest China where the climate is mainly non-humid,and thus the application of CBCs at landfills in Northwest China is promising.It is necessary to evaluate the performance of the CBC that uses loess as a constituent material at liming percolation and landfill gas emissions.Besides,the pore structures of CBCs are sophisticated and susceptible,and present multi-scale heterogeneities due to the influence of complex environmental loadings and climatic conditions.This will significantly affect the water-gas transport properties and practical performance of CBCs.This study focuses on the scientific issue that pore characteristics affect the micro-/macro-scale water-gas transport mechanisms in unsaturated soils.The effects of multi-scale pore heterogeneities on the physical parameters and water-gas transport properties of soils,and the practical performance of CBCs were studied using numerical simulations and theoretical models Based on microfluidic experiments and pore network model,the capillary barrier effect between the fine and coarse pore structures was verified,and relevant influencing factors were investigated.In addition,the field monitoring and numerical simulations were performed to assess the long-term performance of the loess/gravel CBC at limiting percolation and landfill gas emissions in Xi'an city,Northwest China.Based on the obtained results,a procedure for guiding the design and maintenance of CBCs was proposed.The main research work and obtained results of this study are as follows(1)A numerical procedure combining discrete element method and pore network model was presented to efficiently obtain the pore structures and water-gas transport properties of granular soils.This procedure can establish the relationship between the particle geometric attributes and pore structures,and reveal the intrinsic mechanism of how the particle geometric attributes and pore characteristics can affect the water-gas transport properties.The results showed that the particle degradation and shape have significant impact on the porosity,pore geometry and connectivity parameters of soils.The logarithm of air entry pressure decreases linearly with the increasing logarithm of mean pore diameter.Affected by the uniformity coefficient of particles,the fitting parameter m in the van Genuchten equation,which characterizes the slope of soil water characteristic curves,decreases linearly with the increase of the coefficient of variation of pore sizes(COVp):m=-0.48COVp+0.99(2)The effect of micro-scale pore heterogeneity on the water-gas transport properties was explored.A comprehensive analysis of the variations of porosity,intrinsic permeability,spatial distributions of water pressure and flux,flow pattern,soil water characteristic curve,and relative permeability in response to various pore sizes,connectivity,anisotropy and clogging ratios was conducted.The results indicated that,with the increase in the pore size variance,the water flow in soils becomes more non-uniform,the relative water permeability increases and the relative gas permeability decreases.Preferential flows can carry as much as 9.2%of the total flow in soils.Pore clogging has significant effect on the transport parameters of the invading phase,while its effect on the defending phase is negligible.(3)The microscopic formation mechanism of the capillary barrier effect was revealed and its influencing factors were analyzed.Microfluidic chips were used to observe the water imbibition from fine to coarse pore structures under different injection flow rates.Pore network model was used to investigate the effect of pore heterogeneity on the capillary barrier effect.The results indicated that,at the fine-coarse interface,the smaller capillary forces provided by coarse pore structures promote water to flow into fine pore structures which have higher capillary forces.This is the microscopic formation mechanism of the capillary barrier effect.With the increase of the injection flow rate,the invasion pattern of water can be gradually changed from the ordered fingering which dominated by the inertial force to the stable displacement which dominant by the viscous force.The increase in the pore size variance can mitigate the capillary barrier effect.The mean pore diameter ratio of coarse pore structures to fine pore structures should be controlled larger than 5 to ensure the effectiveness of the capillary barrier.(4)A dual-porosity model for coupled water-gas transport considering macro-scale pore heterogeneity in cover soils was proposed.Pore network model was firstly used to define the applicable scope with regard to pore characteristics of the dual-porosity model.Then,several key parameters in the dual-porosity model were determined to characterize the pore heterogeneity of the loess in the field.Finally,the effect of pore heterogeneity on the practical performance of Xi'an landfill CBC was assessed based on the dual-porosity model.The results showed that the dual-porosity model should be adopted for the soils that have apparent macro-pores or those have bimodal distributed pore size and the pore diameter ratio of the second peak to the first is greater than 5.The field loess presents the following parameters in the dual-porosity model:wf=0.2,Kf/Km=100,DE=1.0.The macro-scale pore heterogeneity has slight impact on the percolation of CBC,but it can increase the methane emissions by an order of magnitude.(5)A procedure to enhance the performance of CBCs was proposed.Field monitoring and numerical simulations were carried out to evaluate the long-term performance of the loess/gravel CBC at Xi'an landfill.The results demonstrated that the loess/gravel CBC performed well as a capillary barrier.In the two-year monitoring period from January 2015 to January 2017,the recorded percolation was 16.16 mm,which can meet the percolation limit of 30 mm/yr.Due to the increased water storage induced by the capillary barrier effect,the gas permeability in the loess can decrease by five orders of magnitude in fall,and thus can help the CBC perform better as a landfill emissions controller compared with the loess monolithic cover.Several engineering measures,including increasing the loess thickness,reducing bottom gas pressure,and artificially watering,were recommended to guide the design and maintenance of CBCs.